Jonaz, I've wondered the same thing...I can't speak for your teachers, but mine told me when I asked that silently vibrating the mantra is much more powerful, even though there's nothing "inferior" about chanting aloud or saying japa or soft whispered repetition. Hindus, I have heard, believe the same thing.

My own personal opinion is also that, when you can vibrate them internally without distraction (Even when there are other noises or people speaking) it leads to greater internalization of the mantra or Dharani. This will especially help, in my experience, with recalling the mantra in dreams or in liminal states of consciousness like just waking up. Or even at the time of death.

I think it can depend on the mantra. I was taught that generally mantras for wrathful practices (e.g. protector practices) are not repeated audibly, mantras for semi-wrathful practices are said barely audibly ("so only your collar can hear" - I think was translation of the Tibetan) and peaceful mantras are said audibly but not loudly. Another point that was made to me was that mantras should not be said too fast - each syllable should be distinguishable.

Everyone talks about the weather, but nobody does anything about it - Mark Twain.

Audible mantras act more on the physical level. Improving voice, breathing and physical well being, as well as the aspects of concentration and meditation. Less audibly they give a clear, prescriptive feedback indication of physical and mental health. Inaudibly they allow 'secret' practice and therefore are more subtle and 'advanced'.

Both is a good idea where suitable - when you speak the mantra out loud it also becomes a wholesome action of speech and body.For a healing mantra like Shurangama, out loud can be of benefit to any beings around you like animals etc.